Method and apparatus for spraying waterborne coatings under varying conditions

ABSTRACT

The detrimental effects of fluctuating humidity are neutralized by adjusting the viscosity of a waterborne coating as it is being conducted to the spraying device. A predetermined amount of water or other viscosity modifying additive is mixed continuously with the coating in the coating supply line immediately upstream from the spray device. The amount of water or other additive to be added may be determined by monitoring the humidity in the spray zone. Preferably, an automated feedback control system is employed to adjust the amount of water or other viscosity altering additive being mixed into the coating stream.

This application is a continuation of U.S. patent application Ser. No.08/044,436, filed Apr. 8, 1993, now U.S. Pat. No. 5,916,625.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for applyingwaterborne coating compositions and to the resultant coated articles.

Waterborne coating compositions have become increasingly important todaydue to organic volatile emissions regulations. Waterborne coatingcompositions are utilized to meet these regulations because thewaterborne coating compositions contain much smaller amounts of organicvolatiles than conventional solvent coating compositions.

The application of waterborne coatings is complicated by the fact thatthe evaporation rate of water is dependent on the relative humidity ofthe spray environment. When the percent relative humidity is high (e.g.,greater than 80%) water does not evaporate at a fast enough rate and"wet" paint films are produced. "Wet" films have a tendency to sag, pop,and in cases of metallic pigmented coatings produce dark, mottled filmsthat exhibit areas of non-uniform distribution of the metallic pigment.Sags are unsightly, gravity driven flows that occur on verticalsurfaces, and pops are small pore-like flaws in the coating that arecaused by the violent evolution from the film of trapped solvent (eitherorganic solvent or water). Conversely, when the percent relativehumidity is log (e.g., less than 50%) water evaporates too quickly, and"dry" paint films are produced. "Dry" films tend to be rough and do notexhibit good leveling and flow characteristics.

There have been a number of attempts to overcome these problems, butnone of them have been particularly successful. One approach has been totry to control the atmosphere in the spray booth or spray zone. U.S.Pat. Nos. 3,979,535 and 4,687,686 describe methods of controlling thetemperature and humidity of the air in the spray booth to acceptablepreset levels. If, however, the spray booth is not enclosed and isinstead open to a work room or plant, it may be very difficult andexpensive to control the temperature and humidity in the whole room orplant. Sometimes it is not feasible or desired to enclose the sprayarea.

U.S. Pat. No. 4,132,357 describes a method of applying solvent-thinnedcoatings utilizing a shroud which delivers air at a controlledtemperature and humidity around the atomized spray, forming a controlledlocalized atmosphere. However, ambient air from the spray booth may mixwith this controlled atmosphere as it passes from the spray gun to thearticle being coated, and the controlled atmosphere may dissipate veryrapidly.

Another approach is to add water to the paint film to controlevaporation of water from the film. U.S. Pat. No. 4,344,991 describesdepositing a mist layer of water on top of or beneath the depositedwaterborne coating by simultaneously atomizing water with ancillarynozzles. The water mist is not intended to intermix with the atomizedwaterborne coating. U.S. Pat. No. 4,396,651 describes a coating processin which water is atomized through nozzles ancillary to the main spraynozzle. The atomized water is said to build a uniform water atmospherearound the waterborne coating atomized stream without beinghomogeneously mixed with the coating. Both of these patents describeapproaches that have problems with nozzle configuration complexities andhave problems with accurately controlling the amount of water depositedor added to the constantly changing atmosphere around the atomizedspray.

U.S. Pat. No. 4,341,821 describes spraying an organic solvent onto acoated article either before, after, or at the same time the waterbornecoating is being applied. The solvent is intended to control theviscosity of the waterborne coating to prevent sagging. Spraying solventdefeats the main purpose of using waterborne coatings.

It would be desirable to overcome the problem of applying waterbornecoatings compositions at varying relative humidities while avoiding thedisadvantages of the prior art approaches. In particular, it would bedesirable to avoid batch mixing of coatings with solvents, to avoidcomplicated, non-standard spray gun structures, and to avoid costlyhumidity control schemes.

SUMMARY OF THE INVENTION

It has now been found that the effects of fluctuating humidity can beneutralized by adjusting the viscosity of a waterborne coating as it isbeing conducted to the spraying device. A predetermined amount of wateror other viscosity modifying additive may be mixed continuously with thecoating in the coating supply line immediately upstream from the spraydevice. The amount of water or other additive to be added may bedetermined by monitoring the humidity in the spray zone. Preferably, anautomated feedback control system is employed to adjust the amount ofwater or other viscosity altering additive being mixed into the coatingstream in response to the measured humidity.

In accordance with the present invention, there is provided a method ofapplying waterborne coating compositions onto a substrate under varyinghumidity conditions comprising measuring relative humidity in the sprayarea to which a stream of waterborne coating composition is beingsupplied, using the relative humidity measurement as a basis forselecting the proportions of coating and a viscosity altering additiveto be mixed together in the supply stream, and continuously mixing theselected amount of additive into the coating to obtain an adjustedformulation of the waterborne coating composition which is then sprayedonto a substrate in the spray area.

The present invention accomplishes humidity related formulation changesby an inline method. By "inline" is meant that the waterborne coatingcomposition is adjusted as it moves along the coating supply line as thewaterborne coating is being conveyed to the spray device. Theformulation changes are accomplished continuously rather than by batchadjustment. As a result, there is no waste and little or no lag inadjusting to changing conditions.

Also in accordance with the present invention, there is provided anapparatus for applying waterborne coating compositions to a substratecomprising a spraying device in a spray area, a relative humidity sensorsituated in the spray area, a coating supply line, one or more additivesupply lines, means for controlling proportionate flow rates of thecoating and the additive or additives, means for mixing the additiveinto the coating, control means for receiving an input from the relativehumidity sensor and outputting a signal to the flow proportioning means,a fluid line for directing the mixed coating and additive stream to aspray device in the spray area.

Coated articles prepared in accordance with the invention arecharacterized by their acceptable appearance, being substantially freefrom pops and sags, and exhibiting a smooth surface.

THE DRAWING

FIG. 1 is a schematic diagram of an embodiment of the invention showingan arrangement for carrying out a continuous, humidity-responsivewaterborne coating spray process in accordance with the invention.

DETAILED DESCRIPTION

The present invention will be described with reference to the embodimentdepicted in the drawing, but it should be understood that the inventionis not limited to this particular embodiment.

The present invention encompasses a method of inline adjustment ofwaterborne coatings compositions held, for example, in reservoir 1 tocompensate for the relative humidity of the environment in which thewaterborne coating is being applied in a spray zone 13. The compositionof the coating is not critical; the performance of any commerciallyavailable waterborne coating composition may be enhanced by thisinvention. In one mode of operation in accordance with the presentinvention, the base waterborne coating composition may be formulated forhigh relative humidity conditions, and the viscosity is adjusteddownwardly as relative humidity drops. For this purpose, high humiditymay be considered relative humidities of 80 percent or higher. Usuallywhen a waterborne coating is formulated for application at high humidityonly small amounts of water are used to reduce the coating to its sprayviscosity, or no reduction is used. The viscosity of a reducedformulation for spraying at high relative humidity may be 35 to 60percent higher than the spray viscosity used at low relative humidity.As the relative humidity becomes lower, the formulation can be adjustedby the addition of one or more viscosity altering additives fromreservoir 2 or 3. Alternatively, the base coating composition may beformulated for low humidity and its viscosity adjusted upwardly by theappropriate additive as relative humidity rises. Another variation mayinvolve a base coating composition formulated for moderate humidity, andthe viscosity may be adjusted either upwardly or downwardly withseparate additives.

Most commonly, the viscosity altering additive is a viscosity reducingagent (e.g., water), but the use of viscosity increasing additives isalso encompassed by the present invention. Viscosity altering agentsinclude water, mixtures of water and organic solvent, amines, andmixtures of water and amine. Examples of organic solvents that can beused are glycol ethers such as ethylene glycol monohexyl ether, ethyleneglycol monobutyl ether, diethylene glycol monobutyl ether, propyleneglycol monobutyl ether, or propylene glycol monopropyl ether. Examplesof amines that can be used are dimethanolamine, di-isopropanolamine, ortriethylamine.

The present invention entails measuring the relative humidity of thespray area 13. The spray area 13 may be an open area or closed area suchas a spray booth. By spray booth is meant any enclosed or semi-enclosedspace that has been designed to accommodate the application of coatingsmaterials. The relative humidity measurement can be accomplishedmanually with a sling psychrometer or with any commercially availablehumidity sensor 14. A sensor that is capable of transmitting anelectrical output signal that corresponds to the measured relativehumidity is preferred. For example Model HX93 Humidity and TemperatureSensor manufactured by Omega Engineering, Inc., Stamford, Conn. can beemployed. The Model HX93 uses a thin-film polymer capacitor to senserelative humidity. Other commercial sensors utilizing either a resistivepolymer, a dielectric, or an electrostatic capacitor polymer film can beused.

Relative humidity is the basis for selecting the amount of additive fromreservoir 2 or 3 that is added to the waterborne coating compositionfrom reservoir 1 to compensate for the effects of the relative humidityon the final appearance of the waterborne coating. The additive ispreferably deionized water, but may be a mixture of water and organicsolvent, or a mixture of water and amine. In another envisioned mode ofthe invention the additive can consist of the waterborne coatingcomposition that has been reduced with water to a very low viscosity.Additionally, more than one additive stream can be used.

Control of the proportioning of the additive(s) and the waterbornecoating composition can be accomplished by various means. The controlcan be accomplished manually or by automated means. In one mode, aconstant flow rate of the waterborne coating may be maintained, and theflow rate of the additive may be varied in accordance with theparticular relative humidity. The flow rates can be monitored by flowmeters 4, 5, and 6, which may be any commercially available flow metersuch as a ZHM Series or HPM Series Flow Meter distributed by AW Company,Racine, Wis., or a Model D6 Mass Flow Meter manufactured by MicroMotion, Inc., Boulder, Colo. The ZHM Series and HPM Series meters arepositive displacement, gear flow meters, while the Micro Motion D6 is amass flow meter that measures the vibrations of a U shaped sensor tubeand equates the vibrational forces to fluid forces which areproportional to the mass flow rate. In preferred embodiments,proportional flow control may utilize a commercially availableproportioning system, for example Model 2K™ Dual Component MeteringSystem manufactured by DeVilbiss-Ransburg Industrial Liquid Systems,Inc., Toledo, Ohio. The 2K™ Dual Component Metering System is amicro-processor based analog closed loop fluid control device 12incorporating flow meters 4, 5, 6 and servo valves 7, 8, 9 and providingcontinuous regulation of the fluid flow and mixing process. The unituses ZHM Series flow meters. Preferably the proportioning equipment iscapable of receiving a signal from the humidity sensor that is locatedin the spray booth, and is capable of selecting and setting the propermixing ratio of additive to waterborne coating composition as determinedfor the particular relative humidity. Because the viscositycharacteristics and spraying behavior differ from one coatingcomposition to another, determining the mixing ratios of additive towaterborne coating composition are determined empirically. Varyingamounts of additive are mixed into the waterborne coating compositionand an optimized ratio is determined for each range of relative humidityby choosing the ratio that produces the optimum film that issubstantially free of defects such as sagging, popping, mottling, orexhibiting a dry rough surface that is evidence of poor leveling andflow.

The additive is mixed inline with the waterborne coating composition.Any method of mixing the additive and the waterborne coating willsuffice, but preferably a static mixer 10 is used, for example the 140Series Spiral Mixers manufactured by TAH Industries, Inc., Robbinsville,N.J. Static mixers consist of a series of baffles aligned in a tube. Thebaffles convert the tube into a maze and when fluids are pumped throughthe mixer, they are progressively divided and recombined resulting inefficient, inline mixing.

After mixing the additive with the waterborne coating composition, theadjusted formula can then be sprayed onto the surface to be coated bymeans of any commercially available spray device 11, such as aconventional air atomizing spray device, an electrostatic air atomizingspray device, or an electrostatic rotary atomizing device. The spraydevice may be operated in its normally accepted manner.

The invention is an inline process, not a batch adjustment process. Byadjusting the formulation inline on a continuous basis the ability tospray over a wide range of relative humidities can be accomplished withno waste and no additional inventory.

EXAMPLE 1

A waterborne primer composition was spray applied to test panelsutilizing a rotary atomizer equipped with a charging ring. The rotaryatomizer used was a Behr Bell with an attached charging ring which ismanufactured by Behr Industrial Equipment, Rochester, Mich. Theoperating parameters of the bell were:

    ______________________________________                                        Bell Speed          40,000  RPM                                               Shaping Air         45      psig                                              Voltage             60      KV                                                Bell to Target Distance                                                                           11      Inches                                            Conveyor Speed      11      Ft/Min                                            Paint Flow Rate     180     grams/min                                         ______________________________________                                    

Temperature was held constant to within one degree of 73 degrees F.Experiments were conducted at 40% relative humidity (RH), 60% RH, and80% RH. For each humidity level, test panels were coated with differentamounts of water added continuously to the waterborne primercomposition. At 40% RH panels were prepared with 0, 20, 30, 40, and 50grams per minute of deionized water added inline to 180 grams per minuteof waterborne coating. The primer and the water addition were passedthrough an inline static mixer just prior to entering the Behr Bell. At60% RH panels were prepared with 0, 10, 20, and 30 grams per minute ofdeionized water added inline. At 80% RH panels were prepared with 0 and10 grams per minute of deionized water added inline. Additional panelswere prepared at 40%, 60%, and 80% RH with a pre-reduced controlwaterborne primer composition. The control composition was reduced to24.8 seconds #4 Ford Cup with deionized water.

After spraying, all panels were flashed for five minutes at roomtemperature, then flashed for seven minutes at 175 degrees F, and thenbaked for 25 minutes at 325 degrees F. After baking, the panels werecooled and then film thickness and 60 degree gloss were measured. Alsothe surface appearance was evaluated visually using a scale of: veryrough; rough; smooth; very smooth. Table 1 lists the results.

                  TABLE 1                                                         ______________________________________                                                 Water added                                                                             Thickness  60°                                                                         Surface                                    % RH     grams/min mils       Gloss                                                                              Evaluation                                 ______________________________________                                        40       Control   0.84       61   Rough to                                                                      Smooth                                     40        0        0.90       65   Very Rough                                 40       20        0.68       37   Rough to                                                                      Smooth                                     40       30        0.84       40   Smooth                                     40       40        0.80       56   Smooth                                     40       50        0.71       55   Very                                                                          Smooth                                     60       Control   0.88       62   Smooth                                     60        0        0.91       70   Rough                                      60       10        0.80       55   Rough to                                                                      Smooth                                     60       20        0.80       55   Smooth                                     60       30        0.73       54   Very                                                                          Smooth                                     80       Control   0.86       55   Smooth                                     80        0        0.86       60   Rough to                                                                      Smooth                                     80       10        0.71       62   Smooth                                     ______________________________________                                    

EXAMPLE 2

A waterborne primer composition was applied to test panels utilizing arotary atomizer equipped with a charging ring as in EXAMPLE 1. Theoperating parameters of the bell were:

    ______________________________________                                        Bell Speed          28,000  RPM                                               Shaping Air         40      psig                                              Voltage             60      KV                                                Bell to Target Distance                                                                           12      Inches                                            Conveyor Speed      14      Ft/Min                                            Paint Flow Rate     240     grams/min                                         ______________________________________                                    

Experiments were conducted at 25% RH, 40% RH, 60% RH, and 80% RH. Thewaterborne coating composition flow rate was 240 grams per minute. Thedeionized water addition was varied from 0 to 30 grams per minute. Afterspraying, the test panels were flashed for 5 minutes at roomtemperature, then flashed for 7 minutes at 175 degrees F, then baked for25 minutes at 325 degrees F. After baking, the panels were cooled andthen film thickness and 60 degree gloss were measured. Also, surfaceroughness measurements were made using a #2100 Surfpro Optical Profilermanufactured by A.T.I. Systems, Inc., Madison Heights, Mich. Theinstrument divided the measured surface anomalies into two categories.Anomalies greater than 0.030 inches in width were quantified as orangepeel, and anomalies less than 0.030 inches in width were quantified astexture or micro-roughness. Two readings were returned for eachclassification, frequency and amplitude. The amplitude is the averageheight of the anomaly in micro-inches (10⁻⁶ inches). The frequency is inunits of cycles per millimeter. The results are detailed in Table 2.

                  TABLE 2                                                         ______________________________________                                        H.sub.2 O                                                                              % Wt.   Film   60°                                                                         Orange Peel                                                                            Texture                                 % RH  g/min  Solids  mils Gloss                                                                              Freq.                                                                              Ampl. Freq.                                                                              Ampl.                          ______________________________________                                        25     0     49.97   1.02 33.5 0.84 708   6.94 48                             25    20     45.41   0.90 47.5 0.64 463   8.62 187                            25    30     43.74   1.07 56.5 0.70 140   10.63                                                                              82                             40     0     49.97   1.23 57.5 0.65 400   8.93 199                            40     5     49.51   1.10 60.3 0.64 371   9.17 164                            40    10     47.49   1.26 59.0 0.65 218   10.20                                                                              125                            40    15     46.27   1.22 61.5 0.70 194   10.20                                                                              102                            40    20     45.41   1.29 60.0 0.62 168   10.41                                                                              92                             40    25     45.18   1.01 59.3 0.72 151   9.80 93                             60     0     49.97   1.20 62.1 0.57 231   9.90 114                            60     5     49.51   1.22 63.9 0.62 149   10.74                                                                              78                             60    10     47.59   1.11 62.9 0.64 143   10.20                                                                              80                             60    15     46.27   1.10 64.9 0.71 126   10.98                                                                              71                             80     0     49.97   1.09 61.6 0.75 125   10.74                                                                              83                             80     5     49.51   1.09 62.6 0.75 124   11.11                                                                              76                             ______________________________________                                    

Other variations and modification as would be apparent to those of skillin the art may be resorted to without departing from the scope of theinvention as defined by the claims that follow.

We claim:
 1. A method of controlling the finish quality of waterbornecoatings being spray applied onto a substrate under varying humidityconditions comprising:controlling the evaporation rate of water fromwaterborne coating compositions by: (a) measuring relative humidity inthe spray area in which a stream of waterborne coating composition isbeing supplied to a spray device; (b) based on the relative humiditymeasurement of (a), controlling the proportionate flow rates of thestream of waterborne coating and a separate stream of water to becombined with the waterborne coating composition stream, theproportioning being selected to provide a predetermined evaporation rateof water from the coating under ambient environmental conditions in thespray area; (c) mixing the separate water stream into the waterbornecoating composition stream at the proportionate flow rates selected instep (b) to obtain an adjusted formulation of the waterborne coatingcomposition being conveyed to a spray device; (d) spraying the adjustedwaterborne coating formulation by means of said spray device onto thesubstrate being coated in the spray area.
 2. The method of claim 1 inwhich the spraying is carried out with a air atomized spray device, anelectrostatic air atomized spray device, or an electrostatic rotaryatomizing device.
 3. The method of claim 1 in which the separate streamof water includes an additional additive.
 4. The method of claim 3 inwhich the additive includes an amine.
 5. The method of claim 3 in whichthe additive includes an organic material.